E.coli (Escherichia coli) is fecal-borne bacteria, and when you're talking about toilets, you're in the right neighborhood for contamination. E. coli is a bacteria that's normally found in our intestines, and if you're accidentally exposed to it -- usually from contaminated water or food, but it's been known to cling to nonporous surfaces, too -- you could be struck down with diarrhea (sometimes bloody), abdominal cramping and vomiting.

Gastrointestinal viruses such as norovirus (which is the bane of many a cruise ship crew and passenger), also cause stomach distress, and similar to E. coli, they are easily transmitted from person to person. Norovirus has been found contaminating nonporous surfaces, including toilet seats, for as long as two weeks (and that's despite those surfaces having been cleaned) [source: Said et al].